On Tue, 27 Mar 2007, ? ? wrote:
I¡¯m sorry to trouble you again. In MEEP we could not specify the power
of the source. Rather we could specify the amplitude only. If I want to
set a source with a power 4w, what is the amplitude of the source? Is it
simply obtained by 4^0.5 = 2? However, it seems that the expected
nonlinear effect could not be observed. What¡¯s wrong with it? Many
thanks. Jack
You are thinking of this in the wrong way.
A source in Meep is a current in Maxwell's equations. A given current
source does *not* generate a given power. The radiated power depends on
the time-dependence and on the surrounding geometry --- exactly the same
current, with exactly the same time-dependence, radiates different amounts
of power depending on what its surroundings are. This is a consequence of
the physical laws, independent of Meep's computational method.
So, in a nonlinear calculation where the absolute amount of power is
important, you need to run two calculations. Once to calculate the amount
of power radiated by your source with a given amplitude in the linear
system, and then again to calculate your nonlinear effect (with either the
amplitude adjusted to give your desired power, or your chi3 adjusted to
give the desired index shift).
See also:
http://ab-initio.mit.edu/wiki/index.php/Units_and_nonlinearity_in_Meep
Cordially,
Steven G. Johnson___
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